Inhibition of decomposition of amalgam



Jan. 10, 1961 R. s. KARPIUK 2,967,808

INHIBITION oF DEcoMPosITIoN oF AMALGAM Filed Aug. 14, 1958 I/o/umePercen//n/u'//or INVENTOR. Roef/ .5. /fofol'u TTORNEY INHIBITION FDECOMPOSITION OF AMALGAM Robert S. Karpiuk, Midland, Mich., assignor toThe Dow `Chemical Company, Midland, Mich., a corporation of DelawareFiled Aug. 14, 1958, Ser. No. 755,073

3 Claims. (Cl. 204-99) The invention relates to inhibiting thedecomposition of an amalgam. More particularly, it relates to inhibitingthe decomposition and/or reaction of an alkali metal amalgam during theproduction thereof in the electrolyzer chamber of a chlorine cell whichemploys a mercury cathode in the electrolysis of an alkali metalchloride electrolyte. Reactivity of the alkali metal amalgam duringelectrolysis, although appearing to consist of both decomposition andcomplex reactions with the aqueous electrolyte, will be referred tohereinafter merely as the decomposition thereof for simplicity ofexpression.

A conventional cell and method of producing chlorine is to electrolyzean alkali metal chloride brine, e.g., an aqueous solution of NaCl, toyield chlorine gas at the anode and the alkali metal as an amalgam on amercury cathode, `the amalgam being subsequently decomposed in thepresence of water in a separate chamber or chambers to yield the alkalimetal hydroxide and hydrogen as end products and mercury for re-use inthe electrolysis. metal hydroxide and chlorine employing such cell andmethod, there has heretofore been a loss in ef'liciency, an appreciablepart of which is due to the decomposition admixed with the alkali metalchloride brine employed f in such cells. The compounds which can beemployed in the practice of the invention as additives to such brine yelectrolytes are the alkylene glycols, monoand poly alkylene glycolmonoethers, halo-substituted derivatives of such glycols and ethers,epihalohydrins, and higher morV lecular weight saturated alcohols havingat least 5 car Y bon atoms. Examples of such hydroxy compounds aredipropylene glycol, epichlorohydrin, cyclohexanol, 2 methoxyethanol,2(2-ethoxyethoxy)ethanol, ethylene glycol, Z-ethoxyethanol and ethylenechlorohydrin. Thehydroxyl group is thought to be an essentialsubstituent of the inhibitor employed according to the invention.Epihalohydrins are considered to be within the class because they assumecharacteristics of such hydroxy compounds as the glycols and theirethers, apparently due to hydrolysis at the oxygen bridge. Lowermolecular weight alcohols, however, cannot be employed because of theirtendency to react in a similar manner to, although less violent than,water with an alkali metal amalgam. Hydroxyl-containing compounds havingacidic properties, e.g., carboxylic acids, phenol, cresols, and thelike, cannot be used. The alcohol should contain at least tive carbonatoms to be sufficiently unreactive with the amalgam. The additives,which include both liquids and solids usually exist under normalconditions as liquids f and may be advantageously stirred into theelectrolyte In the electrolytic production of an alkali of the alkalimetal amalgam during the electrolysis. A, mercury cell, of the typedescribed in U.S. Patent.

Attempts have been made to improve the efficiency ofY mercurycathode-type chlorine cells by the addition of inorganic stannates,borates, silicates, or pyrophosphates to the electrolyte. These attemptshave not been suili-` ciently successful in the inhibition of thedecomposition of the alkali metal amalgam to justify their being adoptedgenerally in the production of chlorine employing such cells.

There exists, therefore, a desideratum in the art of chlorine productionby the electrolysis of an alkali metal chloride brine, wherein thealkali metal is recovered as an amalgam on a mercury cathode, for animproved electrolyte and method employing such electrolyte which issubstantially more eilicient due to the inhibition of the decompositionof the alkali metal amalgam during the electrolysis of the brine.

It is an object of the invention to provide such an improved electrolyteand a method of use thereof wherein the decomposition of the amalgambeing formed is suppressed or inhibited thereby improving the efficiencyof the electrolytic production of mercury cathode type cells.

The method and means by which this and related objects of the inventionare attained is made clear in the ensuing description and areparticularly defined in the appended claims.

The invention is an improved electrolyte for and method of producingchlorine and an alkali metal amalgam by the electrolysis of an alkalimetal chloride brine wherein a water-soluble organic hydroxy compound iseither prior to the electrolysis or intermittently or continuouslyduring the electrolytic process to achieve the desired effect of theiruse in accordance with the invention. Each of the additives is aWell-known, readily accessible, and relatively inexpensive material. Theamount of additive to employ is not sharply critical and may becalculated either as a percent by volume or percent by weight based onthe volume or 4weight of brine. For'convenience, the percent by volumeis usually used since the specific gravity of the additive is notgreatly` d iierent from that of the brine. The amount employed in thepractice of the invention is small. At least 0.001 percent byvolumeshould be employed. It is recom-n mended that between 0.005 and 0.03percent by volumek be employed. Greater amounts, e.g., .05 percent ormore maybe employed but are considered uneconor'nical. 0.02 percentappears to be about the optimum amount to add, based on the volume ofthe alkali metal chloride' brine. An apparatus of the type schematicallyshown in',l Figure l was employed to demonstrate the elfects of the-Water-soluble organic hydroxy compounds on the decom-l position rate ofa sodium amalgam when added to alf alkali metal chloride brine in aprimary cell in which an alkali metal amalgam and pieces of graphiteserve as the anode and cathode, respectively, of the cell. De-'composition efiiciency is a measure of the decompositiony of the alkalimetal amalgam in the testing apparatus' used. It is percent, as shown inFigure 2, when no inhibitor is present. Decomposition eieiency isdefined Volume of H2 evolved per second from inhibited brine X100?Volume of H2 evolved per second from o substantially pure brine The rateof decomposition of a prepared sodium amalgamv in a speciicconcentration of sodium chloride brine was ascertained by determiningthe volume of hydrogen` evolved during a measured period of timeemploying the apparatus shown in Figure 1. The decomposition el`` ciencyof the sodium amalgam in the NaCl brine accord-, ing to the invention asthus determined is set outin The apparatus "f 'Figure 1` comprises Pyrexjar 1o.

Patented Jan. 1o, 1961.,

cylindrical dish 12 resting'onthe floor of the jar, sodium amalgam feedtube 14 provided with funnel 16 leading into dish 12 and verticallypositioned graduated cylinder 18 having dome-shaped base 20 resting onthe bottom of jar 1), which forms a gas-collecting means about dish 1-2,5 and having petcock assemblyl 22 and nipple 24 in -the upper partthereof for the attachment of an exhaust means. Sodium amalgam 26A isshown onv the floor of dish 12. In dish 12 and partially submerged inamalgam 26 is graphite anode 28 consisting of two verticallypositioned1/4 plates separated by an insulator. (A single piece of graphite mighthave been employed, but would have been less eicient.) Sodium chloridebrine 30 is shown in Pyrex jar and in tube 14 at the Vsame level as 15in jar 10, and in tube 14 at the same level as in jar 10, and incylinder 18 extending to the top thereof, `tlie brine gam in an alkalimetal chloride bath by the presence of an inhibitor in accordance withthe invention was then shown in the examples below .by following theprocedure and employing the apparatus and the same concentration ofsodium chloride brine as in the conditioning steps above except that ineach of the examples, an increasing amount of an inhibitor was addedtothe brine, according to the invention, contained in jar 10 as shown inFigure l. The amounts of the inhibitor were increased up to 0.02 percentby volume based upon the volume of sodium chloride brine. The effect ondecomposition efiiciency is set out in table form in Table I foradditions of 0.0l and 0.02 percent by volume of inhibitor andgraphically in Figure 2 for the additions shown, up to 0.02 percent. Thelower the decomposition eficiency, in general, the more eiective was theinhibitor employed in the brine.

TABLE I Eeci` on decomposition of amalgam in brine by presence ofinhibitors Properties oi Decomposition Inhibitors Eicency ExampleInhibitor Added to Formula of Inhibitor Percent the Brine Inhibitor Mol.Density by Volume Wt. at C.

1 Dipropylene glycol (CHsCEOH-CHQMO 134. 17 1. 022 51 48 2Epichlorohydrn CHz-CH--CHg-Cl 92. 53 1. 801 64 58 Cyclohexanol CsHnOH100.16 0. 9499 63 63 2methoxycthanol CHa- O-CHz-CHz- OH- 76.09 0. 966 7564 2-(2-ethoxyethoxy) ethanol 01H5-OCH2-CH;-OCH2CH7-O 134. 17 0. 9902 7668 Ethylene glycol HO-CHz-CHz-OH 62. 7 1.1155 76 76 2ethoxyethanolCzH5OCH2-CH2-OH 90.12 0. 9311 88 76 Ethylene chlorohydrm Cl- CHTCHTOH80. 52 1. 213 93 88 no inhibitor Nnna 100 100 being drawn up into 18 byexhausting the air therefrom as explained hereinafter. 4

The practice of the invention will be illustrated by examples showingthe rate of decomposition of an amalgam in a sodium chloride brinecontainingan inhibitor according to the invention in contrast to therate of decomposition of the sodium chloride brine to which no 60inhibitor is added.

To illustrate the practice of the invention, the following procedurewas-followed employing an apparatus of the type shown in Figure 1: Y

The graphite cathode was iirst conditioned by repeat- 55 ing thefollowing series of conditioning steps several times until the volume ofhydrogen gas evolved per minute was stabilized, i.e., was substantiallyuniform:

Graphite cathode 28 was placed in position in dish 12.

A brine consisting of 300 grams of chemically pure sodium chloridedissolved in enough water to make tive liters was placed in jar 10.Petcock 22 was opened and vacuum applied at nipple 24 thereby exhaustingcylinder 18 of ail and causing the brine to rise in cylinder 18, asshown, until it reached petcock 22 at which time petcock 65 22 wasclosed and the vacuum discontinued. The temperature of the brine was 25C.

250 grams of sodium amalgam. consisting of 0.28 percent by weight ofsodium, the balance being mercury, were then poured into funnel 16 anddown tube 14, col- 70 lecting at the bottom of dish 12 as shown.

The rate of hydrogen gas collecting in the graduated upper portion ofcylinder 18 per minute'became stabilized at an average of 0.34milliliter per minute.

The effect on decompositionof an alkali metal amal- A study of Table Iand Figure 2 shows the decomposi- 5 tion eiciency of the sodium amalgamuninhibited brine to` be percent and that additions of variousinhibitors to the pure brine containing the amalgam in accordance withthe invention substantially lowered this efiiciency by inhibiting thedecomposition thereof. Figure 2 also shows that when as little inhibitorwas added as 0.001 percent by volume based on the volume of NaCl brine,the inhibiting effect on the decomposition of the amalgam was readilyapparent and that this effect became. more marked, i.e., thedecomposition eciency continually lessened withincreased amounts of theinhibitor up to 0.02 percent by volume, except for cyclohexanol whichwas somewhat more eiective than the other inhibitors in small amounts(on the order of 0.001 percent) but which leveled oi with a slightlylessening inhibiting eiect at about 0.015 percent. A further` study ofFigure 2 shows that an amount of an inhibitor of at least 0.01 percentby volume should be added, that 0.02 percent appears to be an optimumamount to add, andthat in general some increased inhibiting effectcontinuesbeyond an` amount of 0.02 percent but by extrapolation of thecurves such increased eiectappears inconsequential beyond say 0.03percent, and clearly uneconornical beyond about 0.05 percent by volumeof the alkali metal chloride bath.

Further examination of the table and Figure 2 shows the relativediminution of decomposition of the amalgam when the different inhibitorsare used in accordance with the invention. For example, when 0.02 partby volurne of dipropylene glycol, based upon the volume of the brine, isadded thereto, the decomposition is reduced by 52 percent. The leasteifective of the additives of the asezsos invention employed wasethylenechlorohydrin which showed a diminution of the decomposition ofthe sodium amalgam by 12 percent.

The presence of the OH radical in the organic group appears essential tothe practice of the invention. Furthermore, the presence of two OHradicals such as in the case of the dipropylene glycol, appears toenhance the benecial eiect upon the suppression of the decomposition ofthe sodium amalgam.

The effective inhibition of the decomposition of an alkali metalamalgam, as demonstrated by the example, has direct application when theinhibitor is added to a mercury cathode type chlorine cell employing analkali metal chloride electrolyte.

To illustrate the invention, 0.02 volume percent of dipropylene glycolbased on the volume of the NaCl brine was added thereto in theelectrolyzer chamber of a cell in a commercial chlorine-producing unit.The eiciency of the cell rose 7 percent over its production eiciencyprior to such addition. After 3 days the production eiiciency decreasedto its earlier eciency. 0.02 volume percent of dipropylene glycol wasagain added to the cell upon which the production eiciency again rose by7 percent.

The examples above show that the presence in the brine used in a mercurycathode type cell of a water-soluble organic hydroxy compoundillustrated by alkylene glycols, monoand polyalkylene glycol monoethers,halosubstituted glycols and ethers and such hydroxy-forming compounds asepichlorohydrin, and high molecular Weight saturated alcohols, suppressor inhibit the decomposition and/ or reaction of the alkali amalgamformed at the cathode. Such decomposition during electrolysis, whenuninhibited, decreases the efficiency of the cell to a point where itseriously aiects the output. Markedly raising the eiciency of such cellsby the addition of a readily available, safe, low-cost inhibitor has anextensive economic impact upon chlorine and alkali hydroxide production.

Having described the invention, what is claimed and desired to beprotected by Letters Patent is:

1. The method of producming chlorine by electrolysis of an alkali metalchloride brine employing a mercury cathode consisting of admixing withthe brine between 0.001 and 0.05 percent by volume of a water-solubleorganic hydroxy compound unreactive with alkali metal amalgams selectedfrom the class consisting of alkylene glycols, monoand polyalkyleneglycol monoethers, halosubstituted derivatives of said glycols andethers, epihalohydrin, and higher molecular Weight saturated alcoholscontaining at least 5 carbon atoms.

2. The method according to claim 1, wherein the organic hydroxy compoundis dipropylene glycol.

3. The method according to claim 2, wherein the dipropylene glycol isadmixed in an amount between 0.01 and 0.03 percent by volume based uponthe volume of brine.

References Cited in the iile of this patent UNITED STATES PATENTS1,868,710 McCullough July 26, 1932 2,668,225 Livingstone Feb. 2., 1954FOREIGN PATENTS 465,365 Canada May 23, 1950

1. THE METHOD OF PRODUCING CHLORINE BY ELECTROLYSIS OF AN ALKALI METAL CHLORIDE BRINE EMPOLYING A MERCURY CATHODE CONSISTING OF ADMIXING WITH THE BRINE BETWEEN 0.001 AND 0.05 PERCENT BY VOLUME OF A WATER-SOLUBLE ORGANIC HYDROXY COMPOUND UNREACTIVE WITH ALKALI METAL AMALGAMS SELECTED FROM THE CLASS CONSISTING OF ALKYLENE GLYCOLS, MONO- AND POLYALKYLENE GLYCOL MONOETHERS, HALOSUBSTITUTED DERIVATIVES OF SAID GLYCOLS AND ETHERS, EPIHALOHYDRIN, AND HIGHER MOLECULAR WEIGHT SATURATED ALCOHOLS CONTAINING AT LEAST 5 CARBON ATOMS. 